docs: def make_speaker_utters(utterances: List[Utterance]) -> Dict[str, List[Utterance]]:
      """ Creates a dictionary mapping from speakers to their utterances. """
  
      speaker_utters = defaultdict(list) # type: DefaultDict[str, List[Utterance]]
      for utter in utterances:
          speaker_utters[utter.speaker].append(utter)
  
      return speaker_utters